Advances in semiconductor manufacturing processes, digital system architecture, and wireless infrastructure, among other things, have resulted in a vast array of electronic products, particularly consumer products, that drive demand for ever-increasing performance and density in non-volatile memory. One way of increasing the performance and density of non-volatile memories such as, but not limited to, a flash memory, is to shrink the dimensions of transistors that are used in the form of memory cells in flash memories. It is well-recognized that shrinking the physical dimensions of transistors also reduces the size of the storage media itself and thus reduces the amount of charge that can be stored. One drawback of this approach, however, is such systems are more prone to manufacturing defects and larger cell to cell variations. These manufacturing defects and variations affect memory cells and reference memory cells, thereby reducing operation and reliability of the non-volatile memories.
Conventionally, the way in which a reference cell is selected for a given memory is hardware fixed, for example, by the manufacturer. Therefore, once fixed, the selection of a reference cell cannot be changed. Generally, reference cells are selected for a non-volatile memory based on the technology associated with the non-volatile memory. As such, when the conventionally fixed references are affected by manufacturing defects or cell to cell variation, etc., the entire nonvolatile memory fails because certain operations such as, for example, the read operation with respect to any memory cell in the non-volatile memory may not be carried out successfully.
One possible effect on the reference cell due to manufacturing defects is that the reference cell has high random telegraph noise (RTN). In this case, the reference threshold voltage and/or an active reference current level associated with the reference cell are rendered unstable, This leads to unreliable comparison of these levels with corresponding levels associated with a given memory cell of the memory. As a result, certain operations such as, for example, the read operation with respect to the given memory cell in the non-volatile memory may not be carried out successfully.
Another possible effect on the reference cell due to manufacturing defects or cell to cell variation is that the reference cell becomes less robust and less reliable. For example, assume the reference threshold voltage and/or the active reference current level associated with the reference cell may be programmable. In this case, the reference threshold voltage and/or the active reference current level associated with the reference cell may have to be calibrated to a desired level of, for example, 5V. Finally, the reference threshold voltage and/or the active reference current level should be stable so that certain operations such as, for example, the read operation may be carried out successfully. In this case, when the reference cell has been affected by manufacturing defects or cell to cell variation, an increasing number of over-programming and/or over-erasing operations are required to be performed on the reference cell to stabilize the reference threshold voltage at the desired level of 5V. The over-programming operation may be an operation that increases a measured reference threshold voltage when the measured reference threshold voltage is found to be less than the desired level of 5V, and the over-erasing operation may be an operation that reduces the measured reference threshold voltage when the measured reference threshold voltage is found to be greater than the desired level of 5V. However, performing this increased number of over-programming and/or over-erasing operations to stabilize the reference threshold voltage at the desired level of 5V reduces the integrity and stability of the reference cell, thereby making the reference cell less robust and less reliable.